linux-kernel - Re: [PATCH] arm64: KVM: export current vcpu->pause state via pseudo regs

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Date:	Thu, 31 Jul 2014 16:14:51 +0100
From:	Alex Bennée <alex.bennee@...aro.org>
To:	Christoffer Dall <christoffer.dall@...aro.org>
Cc:	kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
	kvm@...r.kernel.org, Marc Zyngier <marc.zyngier@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: KVM: export current vcpu->pause state via pseudo regs


Christoffer Dall writes:

> On Wed, Jul 09, 2014 at 02:55:12PM +0100, Alex Bennée wrote:
>> To cleanly restore an SMP VM we need to ensure that the current pause
>> state of each vcpu is correctly recorded. Things could get confused if
>> the CPU starts running after migration restore completes when it was
>> paused before it state was captured.
>> 
<snip>
>> +/* Power state (PSCI), not real registers */
>> +#define KVM_REG_ARM_PSCI		(0x0014 << KVM_REG_ARM_COPROC_SHIFT)
>> +#define KVM_REG_ARM_PSCI_REG(n) \
>> +	(KVM_REG_ARM64 | KVM_REG_SIZE_U64 | KVM_REG_ARM_PSCI | \
>> +         (n & ~KVM_REG_ARM_COPROC_MASK))
>
> I don't understand this mask, why isn't this
>             (n & 0xffff))

I was trying to use the existing masks, but of course if anyone changes
that it would be an ABI change so probably not worth it.

>
>> +#define KVM_REG_ARM_PSCI_STATE          KVM_REG_ARM_PSCI_REG(0)
>> +#define NUM_KVM_PSCI_REGS               1
>> +
>
> you're missing updates to Documentation/virtual/kvm/api.txt here.

Will add.

>>  /* Device Control API: ARM VGIC */
>>  #define KVM_DEV_ARM_VGIC_GRP_ADDR	0
>>  #define KVM_DEV_ARM_VGIC_GRP_DIST_REGS	1
>> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
>> index 205f0d8..31d6439 100644
>> --- a/arch/arm64/kvm/guest.c
>> +++ b/arch/arm64/kvm/guest.c
>> @@ -189,6 +189,54 @@ static int get_timer_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>>  }
>>  
>>  /**
>> + * PSCI State
>> + *
>> + * These are not real registers as they do not actually exist in the
>> + * hardware but represent the current power state of the vCPU
>
> full stop
>
>> + */
>> +
>> +static bool is_psci_reg(u64 index)
>> +{
>> +	switch (index) {
>> +	case KVM_REG_ARM_PSCI_STATE:
>> +		return true;
>> +	}
>> +	return false;
>> +}
>> +
>> +static int copy_psci_indices(struct kvm_vcpu *vcpu, u64 __user *uindices)
>> +{
>> +	if (put_user(KVM_REG_ARM_PSCI_STATE, uindices))
>> +		return -EFAULT;
>> +	return 0;
>> +}
>> +
>> +static int set_psci_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>> +{
>> +	void __user *uaddr = (void __user *)(long)reg->addr;
>> +	u64 val;
>> +	int ret;
>> +
>> +	ret = copy_from_user(&val, uaddr, KVM_REG_SIZE(reg->id));
>> +	if (ret != 0)
>> +		return ret;
>> +
>> +        vcpu->arch.pause = (val & 0x1) ? false : true;
>
> tabs

Hmmm, apparently the GNU Emacs "linux" style doesn't actually enforce
that. Who knew? I'll beat the editor into submission.

> I really need the documentation of the ABI, why is bit[0] == 1 not
> paused?

I figured 1 == running, but I can switch it round if you want to to map
directly to the .pause flag.

> If we are not complaining when setting the pause value to false if it
> was true before, then we probably also need to wake up the thread in
> case this is called from another thread, right?
>
> or perhaps we should just return an error if you're trying to un-pause a
> CPU through this interface, hmmmm.

Wouldn't it be an error to mess with any register when the system is not
in a quiescent state? I was assuming that the wake state is dealt with
when the run loop finally restarts. 

<snip>
>
> please check for use of tabs versus spaces, checkpatch.pl should
> complain.
>
> Can you add the 32-bit counterpart as part of this patch?

Same patch? Sure.
>
> Thanks,
> -Christoffer

-- 
Alex Bennée
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